This application incorporates by reference Taiwanese application Serial No. 089111796, filed Jun. 16, 2000.
1. Field of the Invention
The invention relates to an electrode bonding structure for reducing the thermal expansion of the flexible printed circuit board during the bonding process, and more particularly to the electrode bonding structure used in the flat display substrate.
2. Description of the Related Art
A flat display substrate, such as a liquid crystal display (LCD) or a plasma display panel (PDP) substrate, uses several flexible printed circuit (FPC) boards to connect the outer driving circuit and the substrate. The control signals of the driving circuit are transmitted through the FPC board to the substrate. The substrate and the flexible printed circuit boards include a plurality of electrodes to transmit the control signals.
Referring to FIG 1A, 1B and 1C, FIG 1A shows the layout of electrodes in a conventional substrate; FIG 1B is a cross-sectional view along the Y direction of FIG 1A, and FIG 1C is a cross-sectional view along the X direction of FIG 1A. The conventional panel 100 has a glass substrate 102 and the electrodes, such as electrode 104 and 106. These electrodes are bar-shaped and spaced apart from each other on the glass substrate 102. The pitch between the electrodes is P, for example, P can be 300 xcexcm. Besides, the electrodes are made of silver.
The silver electrodes of the conventional panel 100 in FIG 1A have a problem of silver ion migration. This problem becomes more severe as time goes by and eventually resulting in a short circuit. If the resolution of the flat display is increased, the pitch between the electrodes is reduced, then the influence of the ion migration becomes more remarkable.
Referring to FIG. 2, it illustrates the cross-sectional diagram of the substrate and the flexible printed circuit (FPC) board before bonded. The FPC board 200 includes a polyimide layer 202 and a number of electrodes 216. The arrangement of the electrodes 216 on the FPC board 200 is similar to that of the electrodes 218 on the glass substrate 102. Before the bonding process, the sum of all pitches between the electrodes 216 on the FPC board 200 is A xcexcm, and the sum of all pitches between the electrodes 218 on the glass substrate 102 is B xcexcm. Besides, an anisotropic conductive film (ACF, not illustrated) is formed between the FPC board 200 and the panel 100. The ACF further includes a lot of conductive particles for electrically connecting the electrodes 216 and 218 in the vertical direction.
The thermal expansion coefficients of the substrate 102 and the FPC 200 are different because the materials of the substrate 102 and the FPC 200 are different. After the bonding process, the sum of all pitches between the electrodes 216 on the FPC 200 will be changed to Axe2x80x2 xcexcm and all pitches between the electrodes 218 on the substrate 102 will be changed to Bxe2x80x2 xcexcm. In order to bond the FPC and the substrate tightly, Axe2x80x2 must be equal to Bxe2x80x2. The relations between A, Axe2x80x2, B, Bxe2x80x2 are listed below.
Axe2x80x2=Axcex1 xcex1 is the thermal expansion coefficient of the FPC board
Bxe2x80x2=Bxcex3 xcex3 is the thermal expansion coefficient of the glass substrate
Since Axe2x80x2 equals to Bxe2x80x2
B=A xcex1/xcex3 xcex1/xcex3 is the coefficient of thermal expansion compensation
The coefficient of thermal expansion compensation varies according to the materials of the glass substrate 102, the anisotropic conductive film (ACF), the polyimide layer 202, and the materials used for transmitting heat in the process. Since the materials used in the substrate 102 and the FPC board 200 will affect the coefficient of thermal expansion compensation, the design of the FPC board becomes more complicated. Further, in the manufacturing process, the coefficient of thermal compensation will be affected by environmental conditions (e.g. temperature, pressure), resulting in an inaccuracy in connecting all the electrodes 218 on the substrate 102 and all the electrodes 216 on the FPC board 200. Therefore, the quality of the products will be affected.
It is therefore an object of the invention to provide an electrode bonding structure that can reduce the thermal expansion of the FPC board during the bonding process of the FPC board and the display panel. Besides, the silver ion migration problem can be solved by:
1) enlarging the ends of the electrodes and changing the position of the electrodes; and
2) covering the electrodes with a dielectric layer.
The shape of the electrodes on the substrate of the display panel and the FPC board is changed to reduce errors occurring in the bonding process.
The invention provides an electrode bonding structure comprising a substrate, a circuit board, and an anisotropic conduct film ACF. A substrate dielectric layer and an indenting pad are formed on the surface of the substrate, and the indenting pad is lower than the surface of the substrate dielectric layer by a depth H3. The circuit board is placed parallel to the substrate, a circuit dielectric layer and a bump pad are formed on the surface of the circuit board. The bump pad is higher than the surface of the circuit dielectric layer by a height H1. The ACF is placed between the substrate and the circuit board, and has a thickness H2. The position of the indenting pad is corresponded to that of the bump pad. Further, the height H1 is greater than or equal to the sum of the thickness H2 and the depth H3 (H1 greater than =H2+H3) so that the ACF above the bump pad is squeezed after the substrate and the circuit board are bonded for electrically connecting the substrate and the circuit board, and the bump pad is positioned into the indenting pad for reducing a thermal expansion of the circuit board caused during the process for bonding the substrate and the circuit board.
Another electrode bonding structure is also disclosed in the present invention. The electrode bonding structure comprises: a substrate, a circuit board, and an anisotropic conduct film (ACF). A substrate dielectric layer is formed on the substrate and a bump pad is further formed on the surface of the substrate dielectric layer. The top of the bump pad is higher than the surface of the substrate dielectric layer by a height H1. The circuit board is placed parallel to the substrate, a circuit dielectric layer and an indenting pad are formed on the circuit board. The inner surface of the indenting pad is lower than the circuit dielectric layer by a depth H3. The ACF, is placed between the substrate and the circuit board, and has a thickness H2. The height H1 is greater than or equal to the sum of the thickness H2 and the depth H3 (H1 greater than =H2+H3) such that the ACF above the bump pad is squeezed after the substrate and the circuit board are bonded for electrically connecting the substrate and the circuit board, and the bump pad is positioned into the indenting pad for reducing a thermal expansion of the circuit board caused during the process for bonding the substrate and the circuit board.